Method and apparatus for switching, multicasting multiplexing and demultiplexing an ATM cell

ABSTRACT

The present invention pertains to an apparatus for manipulating ATM cells. The apparatus comprises a memory array in which an entire ATM cell can be read or written in one read or write cycle. The apparatus is also comprised of a mechanism for reading or writing the entire ATM cell from or into the memory array. The present invention pertains to a method for switching an ATM cell. The method comprises the steps of receiving the ATM cell at a first input port of a switch from the ATM network. Then there can be the step of storing the ATM cell in one clock cycle in a memory array of the switch. Next there is the step of reading the ATM cell in the memory array in one clock cycle. Next there is the step of transferring the ATM cell from the memory array to a first output port of the switch. Next there is the step of transmitting the ATM cell from the first output port to the ATM network. The present invention pertains to a switch for an ATM cell. The switch comprises I input ports which receive ATM cells from an ATM network, where I≧1 and is an integer. The switch is also comprised of a memory array connected to the I input ports for storing an ATM cell received by one of the I input ports in one clock cycle. The switch also comprises O output ports connected to the memory array, where O≧1 and is an integer. One of the O output ports transmit an ATM cell which is received from the memory array to the ATM network. Additionally, the switch comprises a controller connected to the memory array, I input ports and O output ports for controlling the storage of an ATM cell from one of the input ports into the memory array in one clock cycle. The switch can be used for normal switching operation, multicasting, demultiplexing or multiplexing.

CROSS-REFERENCE

This application is related to contemporaneously filed U.S. patentapplication Ser. No. 08/381,110 titled "METHOD AND APPARATUS FORMANIPULATING AN ATM CELL" by Mahesh N. Ganmukhi and Brian L. Jordan.

CROSS-REFERENCE

This application is related to contemporaneously filed U.S. patentapplication Ser. No. 08/381,110 titled "METHOD AND APPARATUS FORMANIPULATING AN ATM CELL" by Mahesh N. Ganmukhi and Brian L. Jordan.

FIELD OF THE INVENTION

The present invention relates to a memory device and particularly to asemiconductor memory device having on the same integrated circuit devicea mechanism for reading and writing an entire ATM cell into the memoryarray in one read or write cycle. The present invention relates also tothe semiconductor memory device having multiple ports, i.e., multipleinput and output paths, geared towards ATM cell input and output. Thepresent invention relates also to an ATM switch, an ATM cell ratemultiplexer and ATM cell rate demultiplexer.

BACKGROUND OF THE INVENTION

The unit of transmission used in the ATM is a cell. An ATM cell contains53 bytes or 424 bits of information. These cells are transferred at oneof the standard transmission rates, e.g., these cells may be transferredat OC-1 (51.84 megabits/second) or OC-3 (155.52 megabits/second) orOC-12 (622.08 megabits/second) or OC-24 (1.244 gigabits/second) or OC-48(2.488 gigabits/second) and so on. A very high storage capacity and hightransfer (input and output) speed storage devices are very muchdesirable in the ATM network components. The DRAM, Dynamic Random AccessMemory, provide lower cost per bit storage capability and provide morememory in the same unit of area compared to most other semiconductormemory devices. For this reason, DRAMs are excellent choice where largequantities of data need to be stored.

There are number of disadvantages of using a DRAM in a conventional wayto store ATM cells. One can use the commercially available DRAMs tostore the ATM cells but the these DRAMs offer a small number of datainput/output pins, e.g., 1, 4, 8, 16 or 18 bits. Therefore, if onedesires to construct a cell storage device capable of transferring anentire ATM cell at a time, one has to use many such DRAM chips. Forexample, using an 8 data bit wide DRAM one has to use at least 53 DRAMchips in parallel.

The speeds at which DRAMs operate, i.e., read/write the external data,are relatively slow. For example, some of the commercially availableDRAM chips have memory cycle times of 90, 100, 120, 130 nanoseconds,etc. If one decides to use one such DRAM to store (write) or read an ATMcell, 8 bits at a time, then it would take at least 53 write cycles tostore or read the entire cell.

Presently, DRAM memory devices are not commercially available that canread or write an entire ATM cell, in a row of memory array from theexternal world, in one memory cycle.

The present invention provides a capability of reading or writing anentire ATM cell into a DRAM in one memory cycle and therefore provides asolution for high capacity cell storage and high speed ATM cell inputand output with the ATM network external to the integrated circuitdevice.

SUMMARY OF THE INVENTION

The present invention pertains to an apparatus for manipulating, such asbuffering and switching, ATM cells, preferably on an integrated circuitdevice. The apparatus comprises a memory array in which an entire ATMcell can be read or written in one read or write cycle. The apparatus isalso comprised of a mechanism for reading or writing the ATM cell fromor into the memory array. Preferably, the ATM cell is read or writtenfrom external to the integrated circuit device, into the memory array.

The present invention pertains to a method for switching an ATM cell.The method comprises the steps of receiving the ATM cell at a firstinput port of a switch from the ATM network. Then there can be the stepof storing the ATM cell in one clock cycle in a memory array of theswitch. Next there is the step of reading the ATM cell in the memoryarray in one clock cycle. Next there is the step of transferring the ATMcell from the memory array to a first output port of the switch. Nextthere is the step of transmitting the ATM cell from the first outputport to the ATM network.

The present invention pertains to a switch for an ATM cell. The switchcomprises I input ports which receive ATM cells from an ATM network,where I≧1 and is an integer. The switch is also comprised of a memoryarray connected to the I input ports for storing an ATM cell received byone of the I input ports in one clock cycle. The switch also comprises Ooutput ports connected to the memory array, where O≧1 and is an integer.One of the O output ports transmit an ATM cell which is received fromthe memory array to the ATM network. Additionally, the switch comprisesa controller connected to the memory array, I input ports and O outputports for controlling the storage of an ATM cell from one of the inputports into the memory array in one clock cycle.

The switch can be used for normal switching operation, multicasting,demultiplexing or multiplexing.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, the preferred embodiment of the inventionand preferred methods of practicing the invention are illustrated inwhich:

FIG. 1 is a block diagram representation of an apparatus formanipulating ATM cells of the present invention.

FIG. 2 is a block diagram representation of the CVUs, W bus, CDUs andstate machine of the present invention.

FIG. 3 is a block diagram representation of a cell vectorizing unit.

FIG. 4 is a block diagram representation of a cell devectorizing unit.

FIG. 5a is a block diagram representation of a memory sub-array row dataselector and I/O logic.

FIG. 5b is a block diagram representation of a portion of the memorysub-array row data selector.

FIG. 6 is a block diagram representation of a CVU connected to memorysub-arrays.

FIG. 7 is a block diagram representation of CVUs and CDUs connected tomemory sub-arrays through two-way interleaving and piecewise reading andwriting.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings wherein like reference numerals refer tosimilar or identical parts throughout the several views, and morespecifically to FIG. 1 thereof, there is shown an apparatus 10 formanipulating, such as buffering and switching, ATM cells. The apparatus10 comprises a memory array 12 in which an entire ATM cell can be reador written in one read or write cycle. The apparatus 10 is alsocomprised of a mechanism 14 for reading or writing the entire ATM cellfrom or into the memory array 12.

The memory array 12 is preferably comprised of N memory sub-arrays 16,where N≧1 and is an integer. Each memory sub-array 16 has rows 18 andcolumns 20 of memory cells 22, with there being enough memory cells 22to store all the bits of an ATM cell. At least one row 18 or column 20can have enough memory cells 22 to store all the bits of an ATM cell.Preferably, each row 18 is RW bits wide and is greater than or equal tothe number of bits in an ATM cell. Preferably, each column 20 is CW bitswide, where CW is ≧1. Preferably, 384 bits≦RW≦512 bits. Preferably, thememory array 12 is a 4 megabit DRAM having N=16 memory sub-arrays 16 andCW=RW=512 bits. Even though an ATM cell contains 424 bits, theadditional 88 bits may be used for application specific data.

The reading or writing mechanism 14 preferably includes a row reading orwriting mechanism 24 for reading or writing the ATM cell into a row 18of the memory array 12. The reading or writing mechanism 14 can alsoinclude a mechanism 26 for providing cells to the memory array 12. Theproviding mechanism 26 is in communication with the row reading orwriting mechanism 24. The providing mechanism 26 preferably includes a Wbus 32 along which ATM cells travel to the memory array 12.

Additionally, the row reading or writing mechanism 24 can include a rowdecoder mechanism 28 for decoding and selecting a row 18 of the memoryarray 12. The row reading or writing mechanism 24 can include amechanism 30 for selecting a memory sub-array 16 in communication withthe memory array 12 and the row decoder mechanism 28. The selectingmechanism 30 preferably includes a memory sub-array row data selector 44for reading or writing a memory sub-array 16. The W bus 32 is connectedto the selecting mechanism 30.

The memory sub-array row data selector 44 is preferably comprised of Npass gate switch arrays 61. Each pass gate switch array 61 is connectedto a corresponding memory sub-array 16 of the N memory sub-arrays 16.Each pass gate switch array 61 is also connected to the W bus 32 via I/Ologic 63 and to the sub-array address decoder 45. The sub-array addressdecoder mechanism 45 activates the pass gate switch array 61 connectedto the memory sub-array 16 in which the ATM cell is to be written orread. Preferably, each pass gate switch array 61 is comprised of RW passgate switches 62. Each pass gate switch 62 is connected to acorresponding column 20 of the corresponding memory sub-array 16, and tothe sub-array address decoder mechanism 45. The sub-array addressdecoder mechanism 45 substitutes a column decoder in a conventionalDRAM. Note that in FIG. 5a, only one sense of the bit line BL is shownfor the sake of simplicity.

The reading or writing mechanism 14 can include a mechanism 34 fordelivering ATM cells from an ATM network to the W bus 32. The deliverymechanism 34 is connected to the W bus 32. The delivery mechanism 34preferably includes a first mechanism 40 for aligning the ATM cell so itis properly ordered to be written into the memory array 12 when it isdelivered to the W bus 32. The first mechanism 40 is connected to the Wbus 32.

The first mechanism 40 preferably includes a plurality of cellvectorizing units 46 which receive bits of a cell from the ATM network36 and align them so they can be delivered in parallel to the W bus 32.The cell vectorizing units 46 are connected to the W bus 32. The firstmechanism 40 preferably also includes a W-state machine 48 connected tothe cell vectorizing units 46 to control which cell vectorizing unit 46delivers its cell to the W bus 32, as shown in FIG. 2.

Additionally, the reading or writing mechanism includes a mechanism 38for transferring ATM cells from the W bus 32 to an ATM network 36. Thetransferring mechanism 38 is connected with the W bus 32. Thetransferring mechanism 38 preferably includes a second mechanism 42 foraligning the ATM cell so it is properly ordered after it is read fromthe memory array 12 and transferred from the W bus 32 to the ATM network36. The second mechanism 42 is connected with the W bus 32.

Preferably, the second mechanism 42 includes a plurality of celldevectorizing units 50 which receive bits of a cell from the W bus 32 inparallel and align them so they can be delivered in the ATM cell formatto the ATM network 36. (For clarification purposes, the ATM network 36is defined to be the ATM network external to the chip when the apparatus10 is on a chip.) The cell devectorizing units 50 are connected to the Wbus 32 and the state machine 48. The state machine 48 controls whichcell devectorizing unit 50 receives a cell from the W bus 32.

Each cell vectorizing unit 46 is comprised of at least one layer 54 of aplurality of registers 56 in series which receive the bits of an ATMcell, as shown in FIG. 3. Each cell vectorizing unit 46 is alsocomprised of a cell vectorizing state machine 58 connected to theregisters 56 and the state machine 48. The cell vectorizing statemachine 58 controls which bits of the ATM cell go to which register 56,determines when all the bits of an ATM cell are stored in the registers56, and drives the bits of the ATM cell in the registers 56 to the W bus32 when the state machine 48 instructs it to do so.

Preferably, each cell devectorizing unit 50 is comprised of at least onelayer 55 of a plurality of registers 56 in series which receive the bitsof an ATM cell from the W bus 32, as shown in FIG. 4. The celldevectorizing unit 50 is also comprised of a cell devectorizing statemachine 60 connected to the registers 56 and the state machine 48. Thecell devectorizing state machine 60 controls which bits of the ATM cellgo to which register 56, determines when all the bits of an ATM cell arestored in the registers 56, and drives the bits of the cells in theregisters 56 to the ATM network 36.

The W bus 32 is preferably comprised of RW bus lines 33. Each bus line33 is connected to a corresponding I/O logic 63 in each pass gate switcharray 61. Each register 56 of the one layer 54 of the cell vectorizingunit 46 and the cell devectorizing unit 50 is an 8-bit register having 8register cells 57, as shown in FIG. 6. Each register cell 57 holds abit. Each register cell 57 is connected to a corresponding bus line 33of the W bus 32.

In the operation of the invention, an ATM cell from an ATM network 36 isreceived by a CVU 46a, as shown in FIG. 1. The ATM cell on the network36 is directed to the CVU 46a from a corresponding ATM physical layerpart (not shown). The CVU state machine 58 is connected to the ATMnetwork 36 and operates based on the ATM network 36 interface timing.This timing is identified to the CVU state machine 58 through a RCLKsignal which is received by CVU state machine 58. The RCLK signal isproduced from the ATM physical layer interface. The components of theapparatus 10, such as the CVU state machine 58 and the registers 56 inthe CVU 46 operate based on the timing of the ATM network 36.Preferably, they operate based on the rising edge of the RCLK signal.

There is received by the CVU state machine 58, in conjunction with thefirst byte of the cell is being received by a first register 56a of theCVU 46a, an RSOC signal. The RSOC signal is the receive start of cellsignal that identifies to the CVU state machine 58 that the first byteof the cell has arrived at the CVU 46a. The RSOC signal is active at thesame time the first byte of cell data on a RDAT signal is received. TheRSOC signal is inactive while the other bytes of the cell are receivedby the CVU 46a. It is ignored when the RWENB signal is inactive.

The RDAT signal, or the receive data signal corresponding to the ATMcell data that is arriving from the ATM network 36, is a signal thatcarries the data that is written into the CVU 46a. The RDAT signal issampled on the rising edge of the RCLK signal when the RWENB signal isactive and the RCA signal is true. Each register 56 and the CVU statemachine 58 of the CVU 46 receives the RCLK signal for timing purposes.It should be noted in this context, ATM cell data means header as wellas payload, although it could be just the payload.

While the CVU 46 is receiving the ATM cell, the RWENB signal is alsoreceived by the CVU state machine 58. The RWENB signal is the receivewrite enable and is active when it is low. The RWENB signal is used toindicate on which rising edges of the RCLK signal the CVU 46a shouldaccept another byte of data on the RDAT signal. Thus, the RWENB signalindicates that there is a bit of a cell to be received by the CVU 46. Inorder for the bit to be received, not only does the RWENB signal need tobe active low, which means that there are bits to be stored in the CVU46a, but an RCA signal which is sent to the external device that issending the ATM cell and the RSOC signal, the RDAT signal, the RWENBsignal and the RCLK signal, must have indication from the RCA signal ofthe CVU state machine 58 that the CVU is accepting cell data. The RCA isa receive cell available signal. When the RCA signal is high or activeanother byte of cell data will be accepted by the CVU 46. It is low orinactive when writing another byte of data will overwrite data from aprevious cell that has not been written to the memory array 12 yet. Allof these signals operate on a per clock cycle basis and thus each byteof data is under specific control as to whether it should be entered tothe CVU 46 or not. Thus, the RCLK signal provides timing for the CVU 46,the RSOC signal identifies the first byte of an ATM cell that arrives(this is helpful in alignment and timing purposes also since if someportion of ATM cell is lost, the CVU can identify when to start overagain with a new ATM cell by the RSOC signal), the RWENB signal alertsthe CVU 46 that there are bytes of a cell ready to be sent, and the RCAsignal indicates that the CVU 46 is receptive to receiving a cell. TheRDAT signal is essentially the data of the ATM cell.

The CVU 46a has a first register 56a which is an 8 bit register, asshown in FIG. 3. The first register 56a receives the first 8 bits of thecell. The first register 56a is in communication with the CVU statemachine 58. The CVU state machine 58 controls the operation of the firstCVU 46a through the signals, such as the RCA signal.

When the first register 56a has received 8 bits from the ATM cell, theCVU state machine 58 directs the 8 bits in the first register 56a to bereleased and transferred to a second register 56b. At the same time, theCVU state machine 58 records the fact by incrementing an internalcounter that the first 8 bits of the ATM cell have been transferredthrough to the second register 56b.

After the first 8 bits of the ATM cell have been transferred from thefirst register 56a to the second register 56b, the next 8 bits of theATM cell are received by the first register 56a. Then, the CVU statemachine 58 releases the second 8 bits of the ATM cell and transfers themto the third register 56c. At the same time, the CVU state machine 58records the fact by incrementing the internal counter that the next 8bits of the ATM cell have been transferred to the third register 56c.This operation repeats itself until all the registers 56 in the secondlayer 54b in the first CVU 46a have been filled. After the registers 56in the second layer 54b are filled with bits of the ATM cell, in thenext clock cycle, all the remaining bits in all the registers 56 of thesecond layer 54b are transferred at the same time to the correspondingregisters 56 in the first layer 54a of registers. The remaining bits ofthe ATM cell are written into the remaining registers 56 of the firstlayer 54a, as described above. In this way, when the bits of the entireATM cell are in the registers 56 of the first layer 54a, the CVU 46a cancontinue receiving a new ATM cell by storing its bits in the registers56 of the second layer 54b. As long as the registers 56 of the firstlayer 54a have released their bits before the registers in the secondlayer 54b are filled, the CVU 46a can operate essentially continuouslyas described above.

The external controller 110 provides address inputs to an address buffer112. The address buffer 112 is connected to each CVU 46 and each CDU 50.As each CVU 46 has its registers 56 filled with the bits of an ATM cellfrom the network 36, an address register 114 and the CVU state machine56 in the respective CVU 46 receives the address information from theaddress buffer 112.

When the CVU state machine 58 receives the instructions from the W-statemachine 48 to release the bits of the ATM cell from the registers 56,not only do the bits go to the respective bus lines 33, but the addressregister releases its bits to the address bus 122 that is connected tothe row address buffer 70 and the memory sub-array row data selector 44.The address information that travels on the address bus 122 identifieswhere in the memory array 12 the bits of the ATM cell are to be stored.The address information has three parts to it. It has row address,sub-array address and port number. Assuming a 4 megabit DRAM, with 16sub-arrays, the address will look like: Row address of 9 bits andsub-array address of 4 bits and port number of 3 bits.

Each CVU 46 and CDU 50 is hard coded with a port number. These encodingsare as follows.

    ______________________________________                                                000          CVU0                                                             001          CVU1                                                             010          CVU2                                                             011          CVU3                                                             100          CDU0                                                             101          CDU1                                                             110          CDU2                                                             111          CDU3                                                     ______________________________________                                    

Along with the address information, the external controller 110 providesan address₋₋ strobe signal 133. Additionally, there is a chip₋₋ selectinput 132. The chip₋₋ select signal along with the address₋₋ strobesignal 133 controls the selective operation of the CVUs or CDUs. Theexternal controller 110 sends the address information and the chip₋₋select signal and then sends the address₋₋ strobe signal. All the CVUsor CDUs compare their own port number to that in the addressinformation. If the two port numbers match, then the address is writteninto the local address register 114 by the address₋₋ strobe, providedthe chip₋₋ select signal 132 is active. If the port numbers match andthe chip₋₋ select 132 is not active, then the currently received data inthe ATM cell is rejected and the CVU or the CDU starts processing thenext ATM cell.

A CVU 46 asserts the cell₋₋ ready signal only when the entire ATM cellis constructed in the first row 54a of registers 56 and the address isavailable in the address register 114. The cell ready signal indicatesto the W-state machine 48 that the ATM cell data and address isavailable. A CDU 50 asserts a ready₋₋ for₋₋ cell signal when the addressis available in the address register 114. The ready₋₋ for₋₋ cell signalindicates to the W-state machine 48 that the ATM cell data is neededfrom the address location.

When the first layer 54 of registers 56 are filled with all the bitsfrom the ATM cell and the address register 114 in the CVU 46 hasreceived the address of the location where the ATM cell is to be storedin the memory array 12, the CVU state machine 58 informs the W-statemachine 48 that the ATM cell stored in the first CVU 46a is ready fortransfer to the memory array 12. The CVU state machine 58 knows the CVU46a is filled with all the bits of the ATM cell since it has kept countof the bits that have passed through the first register 56a and knowshow many bits are in an ATM cell, as well as the CVU state machine 58knows that the proper address is received.

When the CVU 46 has stored an entire ATM cell in the registers 56 of thefirst layer 54a, the CVU state machine 58 sends a cell ready signal tothe synchronizer 120. The synchronizer 120 synchronizes the cell readysignal from RCLK domain to WCLK domain. The WCLK signal is a timingsignal from and controlled by the external controller 110. Thesynchronizer is necessary so that the various signals passed between theW-state machine 48 and the CVU state machine 58, which operate atdifferent clock frequencies, can be understood by each other. Thesynchronizer 120 after it receives the cell ready signal from the CVUstate machine 58 sends a synchronized cell ready signal to the W-statemachine 48. When the W-state machine 48 receives the sync cell readysignal from the synchronizer 120, informing it that the CVU 46a is readyto release its cell to the W bus 32, the W-state machine 48 determineswhether it is the appropriate time for the transfer of the ATM cell fromthe CVU 46a to the W bus 32. When the W-state machine 48 deems itappropriate to transfer the ATM cell from the CVU 46a to the W bus 32,the W state machine 48 sends a dequeue cell signal to the synchronizer120. The synchronizer 120 takes the dequeue cell signal from the W-statemachine 48 and synchronizes it to the RCLK. The synchronizer 120 thensends a sync dequeue cell signal to the CVU state machine 58 informingthe CVU state machine that the bits from the first layer 54 of registers56 have been used and the first layer 54 of register 56 is now ready toreceive data from the next ATM cell.

The CVU state machine 58 also receives a T-count signal whichessentially identifies the length of a cell it is going to receive, amode signal from the mode register 126 which identifies how the CVU 46corresponding with the CVU state machine 58 is to operate, and a resetsignal to clear and initialize the CVU state machine 58.

As mentioned above, in the event another ATM cell arrives at the firstCVU 46a before the first ATM cell stored in the first layer 54 ofregisters 56 have been released to the W bus 32 for transfer to thememory array 12, there is a second layer 54b of registers that areseparated from the W bus 32 by the first layer 54a of registers 56. Byhaving the second layer 54b of registers 56 present, it prevents ATMcells from being lost that come to CVU 46a because there is no place forsuch a cell to be stored. This second layer 54b of registers 56, as wellas any additional layers 54 that may be required in order not tooverflow the CVU 46 is given by the following. ##EQU1## where b is thenumber of bits that arrive in each RCLK clock period at the CVU 46,T=DRAM read/write cycle clock period, t=interface clock, RCLK, period,N=number of CVUs and CDUs and refresh=refresh time needed in terms of T;generally 1. The above number is referred to as secondary storageregister number. There is an assumption in the above equation that therefresh cycles are evenly distributed over the refresh time. If a burstof refresh is performed, the secondary storage register number becomestoo high.

As cells arrive at the various CVUs 46, the filling of the CVU 46 isrepeated as described above. As the CVUs 46 receive address and cellsand are aligned and ready to transfer their cells to the W bus 32 forultimate delivery to the memory array 12, they are continuouslyinforming the W-state machine 48 of their status. The W-state machine 48maintains the organization of when the cells that are ready to betransferred to the W bus 32 are actually transferred by controllingtri-state enable so there is no competition or corruption of the cells.The W-stage machine 48 arbitrates between the CVUs and CDUs that arecompeting for the cell transfer over the W bus by an arbitration scheme,for example, a simple round robin scheme. When the W-state machine 48decides that it is time to transfer a cell from a CVU 46 to the memoryarray 12, the W-state machine 48 enables the tri-state buffers ofaddress and data, and also generates RAS, CAS related clock signals. TheW-state machine causes the CVU 46 to release its cell to the W bus 32 insuch a manner that cells arriving at the CVUs 46 from the ATM network 32always have a place to be stored in a CVU 46 because there can be a cellbeing transferred from a CVU 46 to the W bus 32 as another cell isarriving at a CVU 46 from the ATM network 36.

The ATM cell after it is transferred to the W bus 32, travels along theW bus 32 to the I/O logic and then to the memory sub-array row dataselector 44. The memory sub-array row data selector 44, as shown in FIG.5a, is comprised of 16 pass gate switch arrays 61, each of which has 512(RW) pass gate switches 62. Each pass gate switch 62 is connected to abit line of a sub-array 16. The memory array 12 is a DRAM such as aToshiba TC524162 DRAM having a size of 4 megabit (16×512×512 bits). Asthe ATM cell arrives at the memory sub-array row data selector 44, eachbit is essentially traveling in parallel with the other bits of the cellalong the W bus 32 until each bit goes to a corresponding pass gateswitch 62. Thus, the first bit of the ATM cell goes to the first passgate switch 62a, the second bit of the ATM cell goes to the second passgate switch 62a and so on for all the bits of the ATM cell.

The row decoder mechanism 28 is comprised of a row address decoder 68that decodes the row address and therefore selects a row 18 in eachsub-array 16 for purposes of storing the cell in a memory cell 22 in thedesired row 18. There is also a row address buffer which drives the rowaddress of interest; and a refresh counter 72, refresh controller 73connected to the refresh counter 72 and the W-state machine 48, and arefresh timer 75 that is clocked by the WCLK signal connected to theW-state machine 48 to provide the refresh to the memory cells 22. Theoperation of the row address decoder 68, the row address buffer 70, therefresh counter 72 the refresh controller 73, and the refresh timer 75,are well known in the art. See U.S. Pat. No. 5,305,280, incorporated byreference, for description of the ROW decoder and "Dynamic RAMs andMemory Modules" databook by Motorola, Inc., 1994, incorporated byreference. Essentially, the selection and decode of a row 18 isperformed as in a conventional DRAM.

The sub-array decoder 45 receives the sub-array address and it decodesthe sub-array address. The information from the sub-array addressdecoder is connected to the memory sub-array row data selector 44. Thesignals from sub-array address decoder 45 are essentially connected toeach pass gate switch 62 to place an enable on signal onto each desiredpass gate switch 62 to allow the bit from the I/O logic to pass throughthe pass gate switch 62 to the bit lines of a corresponding sub-array 16where the bit is ultimately stored in a memory cell 22. Thus, the I/Ologic passes its respective bit to a corresponding pass gate switch 62for each sub-array 16. The sub-array 16 and specifically the memory cell22 in the sub-array 16 that the bit ultimately is stored in isdetermined by which pass gate switch 62 of which pass gate switch array61 receives an enable on signal from the sub-array address decoder 45 toallow the bit to transfer through the pass gate switch 62 by beingprovided to the bit lines of a memory sub-array 16. After the bit passesthrough the pass gate switch 62, it goes to a sense amplifier 74corresponding to a sub-array 16 and ultimately to the memory cell 22where it is to be stored as is well known in the art. Essentially, thecolumn decode operation in a conventional DRAM is replaced by the memorysub-array row data selector 44. Instead of a column address as is foundin a conventional DRAM, there is a sub-array address provided. Thisoperation is done for every bit of the ATM cell and thus the entire cellis stored in the memory array 22 along a given row 18.

When the bits of the ATM cell arrive at the memory sub-array row dataselector 44, each of the bits go to their respective I/O logic and toeach of the pass gate switch arrays 61 and specifically to acorresponding pass gate switch 62 therein. The sub-array address decoder45 has activated only the desired one of the pass gate switch array 61which are connected to the corresponding sub-array 16 where the bits areultimately written. If all the bits of the ATM cell are to be writteninto a single sub-array 16, then none of the other pass gate arrays 61are activated by the sub-array address decoder 45 so none of the bitsare written in the wrong memory cell 22 in the memory array 12.

All the columns of the memory sub-array row data selector 44corresponding to a memory sub-array 16 have a single common controlsignal, for instance, a, b, c . . . etc. which, when received, enablesthe data corresponding to that memory sub-array 16 to be sourced on theI/O bus through the pass gate array 61. In this way, a single controlsignal activates the entire portion of the memory sub-array row dataselector 44 that connects to the corresponding memory sub-array 16, asshown in FIG. 5a so the ATM cell can be written into the memorysub-array 16. FIG. 5b shows a schematic representation of a single passgate switch array 61 of the memory sub-array row data selector 44connected to a memory sub-array 16 of the memory array 12.

Integrated circuit memories generally contain a two-dimensional array ofstorage cells arranged in rows and columns. A common architecture is toconnect all cells in a row to a common row line, often referred to asthe "word line" and all cells in a column to a common column line oftencalled the "bit line" or "digit line". In this architecture, the rowline provides a signal which enables cells to receive or output a datasignal and the column line provides the input or output line on whichthe signal is transferred. An individual cell is addressed via a rowdecoder that selects a row to be addressed and a column decoder whichselects a column to be addressed, thereby selecting one particular cellat the corresponding row and column location. The cell is accessed byplacing an enable signal on the row line in the row associated with thecell and reading or writing a signal on the column line associated withthe cell.

In regard to the sense amplifier 74, integrated circuit memories arealso generally binary logic circuits in which information is stored andtransferred as voltages representing complementary logic values that arealternately referred to as "true and false" "logic 1 and logic 0" or"logic high and logic low". Typically, a voltage of 5 volts mayrepresent the logic 1 state while a voltage of zero volts represents thelogic 0 state. Because of the constraints of resistance, capacitance,etc., the individual voltages input to or output on the column lines byindividual cells are usually at some intermediate voltage. Thus,subcircuits are associated with the column lines of integrated circuitmemories to pull the high voltage values up to, or as close as possibleto, the full logic 1 voltage, for example, 5 volts, and to pull the lowvoltages down to as close to the logic 0 voltage, for example, 0 volts,as possible. These subcircuits are commonly referred to as senseamplifiers. See pages 143 to 161 of Circuits, Interconnections, andPackaging for VLSI by H. B. Bakoglu, Addison-Wesley Publishing Company,Inc., 1990; and application note 53 in Memory Products Data Book Volume1 of 2 (Document No. 60105-1-V1) by NEC Electronics, Inc. 1993, both ofwhich are incorporated by reference.

In a read operation, the bits of an ATM cell in a row 18 essentiallyfollow the reverse process for writing or storage. In the readoperation, the bits in a row 18, i.e., 512 bits of data, from each ofthe 16 sub-array 16 is muxed and only 1 of the 16 rows 18 are selectedand provided to the W bus 32.

After all the bits of a cell are read from the memory array 12, they areprovided back to the W bus 32 where they travel to the celldevectorizing unit (CDU) 50. The CDU 50 they go to is dictated by theW-state machine 48 which is connected to each CDU 50. When the CDU 50receives the cell, the cell is segmented into 8 bits and placed intoregisters 56 in the CDU 50. Registers in register 56 are grouped as8-bit registers.

When CDU 50 receives an address from the external controller 110 to readdata from a particular location, it is ready to receive an ATM cell.When a CDU 50 is ready to receive a cell, the CDU state machine 60 sendsa ready₋₋ for₋₋ cell signal to synchronizer 120. The synchronizer 120also receives a TCLK signal identifying the timing of the ATM network 36interface which is the basis for the timing of the CDU state machine 60.The synchronizer 120 takes the ready₋₋ for₋₋ cell signal and the TCLKsignal and produces a sync ready for cell signal, synchronized to WCLK,that is sent to the W-state machine 48. The synchronizer also receives aWCLK signal that is produced by the WCLK signal that identifies thetiming of the W bus 32 and the W-state machine 48. As explained above,the synchronizer 120 synchronizes signals between the two differentclock domains so that the CDU state machine 60 and the W-state machine48 can understand the signals they send to each other.

After the W-state machine 48 receives the sync ready for cell signalfrom the synchronizer 120 of the CDU 50, when the W-state machine 48determines by the arbitration scheme, it causes the address informationfrom the address register 114 of the CDU 50 to be provided to row dataselector 44 and the row decoder 68. As explained above, the memorysub-array row data selector 44 and the row decoder 68 cause the bits ofthe ATM cell that is desired to be transferred to the CDU 50 to be readout of the memory sub-array 16 in which the cell is stored. The transferof the bits of the ATM cell from the memory sub-array 16 is the oppositeprocedure that occurs for a write operation as described above. Theaddress register 114 has previously received the address informationfrom the external controller 110. The address information travels to theaddress register 114 and to the memory sub-array row data selector 44and the row decoder 68 and row address buffer 70 by way of the addressbus 122. The address register 114 and the CDU state machine 60 bothreceive the address information.

When the bits of the ATM cell are read from the memory sub-array 16,they are transferred to the respective bus lines 33 of the W bus 32 andtravel to the CDU 50. At the same time the bits of the ATM cell aretraveling on the W bus 32, the W-state machine 48 has sent a cell₋₋ is₋₋written signal to the synchronizer 120. The synchronizer 120 produces async₋₋ cell₋₋ is₋₋ written signal that is corresponding to the TCLKtiming of the ATM network 36 interface. By being converted to thistiming, the sync cell is written signal is understood by the CDU statemachine 60 when it receives a signal. At the same time, the cell iswritten signal is also provided to the respective registers 56 in thefirst layer 54 of registers 56 of the CDU 50. The cell is written signalserves to activate these registers 56 in the first layer 54a so that thebits on the W bus 32 are all read into the respective register cells 57of registers 56. Since each register cell 57 is connected to a uniquebus line 33 of the W bus, all the bits of the ATM cell are written tothe respective register cells 57 at the same time.

After the CDU state machine 60 has selected the bits from the registers56 of the first layer 54a to the point where the remaining bits in theregisters 56 of the first layer 54a correspond to the registers 56 ofthe second layer 54b, all the remaining bits and all the remainingregisters 56 of the first layer 54a transfer in the same clock cycle tothe corresponding registers 56 and the second layer 54a. In this way,the entire first layer 54a of registers 56 becomes clear to receive thenext ATM cell. At the same time, the select signal from the CDU statemachine 60 still continues to receive the remaining bytes in their orderof the ATM cell, but now the select signal is reading the bytes from thesecond layer 54b of registers 56. After the remaining bits of the cellin the first layer 54a of registers 56 is transferred to the secondlayer 54b of registers 56, the CDU state machine 60 sends the ready forcell signal to the synchronizer 120 to begin the process again, providedit has received a new read address. The length of the first layer 54a ofregisters 56 minus the second layer 546 of registers 56 can be computedby the formula mentioned earlier.

After all of the bits of the ATM cell are stored in the registers 56 ofthe first layer 54a, the CDU state machine 60 begins at the first byteof the cell in the register 56 connected to a 64 to 1 by 8 mux 123. TheCDU state machine 60 sends a select signal which causes the mux to readthe first byte of the ATM cell and passes it to a register 129 to formthe TDAT signal. The register 129 also receives the TCLK signal and anenable signal which only enables the register 129 when the TRENB signalis active and the TCA signal is active. Each clock cycle of the TCLKsignal causes the CDU state machine 60 to produce the select signal forthe next byte in the ATM cell, provided TRENB and TCA are active. Thenext byte passes through the mux 123 to the register 129. Since thisoccurs each clock cycle in regard to each byte, there can be controlover the movement of each byte. In regard to the aforementioned signals,the TCLK signal is supplied from the external device and all othersignals in the CDU are synchronous to the rising edge of this TCLKsignal. The TRENB signal is a transmit read enable signal that indicatesthat the external device is receptive and ready to receive the ATM cellbyte. The transmit read enable or TRENB signal is active when it is low.It is used to indicate on which rising edges of the TCLK signal the CDU60 should issue another byte of data on the TDAT signal. When the TRENBsignal is sampled low by the TCLK signal, another byte of data will beavailable on the next rising edge of the TCLK signal. At maximumtransfer speeds, the TRENB signal will be low for the entire duration ofa cell transfer and a new byte of data is available on every rising edgeof the TCLK signal. The TRENB signal is ignored when the TCA signal islow. The TCA signal is produced by the CDU state machine 60 andidentifies that there is cell available for transmission by the CDU 60.There is also a TSOC signal that is sent by the CDU state machine 60.The TSOC signal indicates a transmit start of cell by the CDU 60. TheTSOC signal is high at the same time as the first byte of a cell isavailable on the TDAT signal. It is useful for timing purposes in theevent that there is error in the transmission of a cell, and theexternal device knows when a new cell is beginning to be transmitted.The TDAT signal is produced by the CDU 50. The TDAT signal is the datathat is transmitted and has the ATM cell bits. The data is written tothe register 129, and sampled on the rising edge of the TCLK signal whenthe TRENB signal is active (and the TCA signal is high).

In one embodiment, each register cell 57 of a register 56 of a firstlayer 54a is connected to a corresponding bus line 33, as shown in FIG.6. If there are 512 bus lines 33, one bus line for each bit of the ATMcell, then there is a unique bus line 33 for each register cell 57 ofeach register 56 of the first layer 54a of a CVU 46.

In one embodiment, memory bank interleaving may be used. Memory bankinterleaving is well understood and often used by those skilled in theart. In two-way interleaving, the memory array 12 is divided into twobanks, each containing either 16 half (256 rows by 512 columns) memorysub-arrays or 8 (512 rows by 512 columns) memory sub-arrays. The firsthalf of ATM cell (24 bytes) from CVUs is written into one memory bankand the second half of ATM cell (24 bytes) from CVUs is written intoanother memory bank. This is done in such a way that the two memorycycles overlap with one another without conflicting the data on the Wbus 32. This kind of two-way memory interleaving has an advantage thatit allows for W bus 32 width reduction by half (256 bits). If four-wayinterleaving is designed, then the width of the W bus 32 can be reducedtwo-fold (128 bits). The memory bank interleaving has also the advantageof hiding the memory precharge time.

Another way memory interleaving can be designed to increase thebandwidth of the W bus 32. In this case, assuming two-way interleaving,the memory array 12 is divided into two banks, each consisting of 8 (512rows by 512 columns) memory sub-arrays 16. The W bus 32 width is notreduced but the W bus bandwidth is increased up to twice the originalbandwidth. In this case, complete ATM cells are read or written in thealternate banks.

In all the above cases of interleaving memory banks, additional burdenis placed on the W-state machine 48. The W-state machine 48 has toprovide separate RAS, CAS, etc. clock signals to each memory bank, alsoit has to provide separate address lines to row decoder 68 and sub-arrayaddress decoder 45. The W-state machine 48 has to control the dataoutput each bank and also the CVUs such that data conflicts do not occuron the W bus 32. There are many other ways of designing the memory bankinterleaving. Some of these can be found in the reference mentionedbelow.

If interleaving is utilized, then a register cell 57 may be connected toa bus line 33 to which at least one or more additional register cells 57are connected. If two-way interleaving is utilized, then there may betwo register cells 57 connected to each bus line 33, there being only256 bus lines 33 instead of 512 bus lines, as shown in FIG. 7. Iffour-way interleaving is used, then there can be four register cells 57connected to each bus line 33, there being 128 bus lines 33. Wheninterleaving is used, then the state machine 48 controls the respectiveCVU state machine to only allow certain registers 56 to release theirbits of the ATM cell at the proper time so all the bits of the ATM cellare still provided to the memory array 12, but they are released overtime instead of all at once to the bus lines 33 of the W bus 32.

If interleaving occurs, then all the bits can still be written throughone pass gate switch array 61 corresponding to one memory sub-array 16.In the event of interleaving, since there may be fewer bus lines, therealso are correspondingly fewer I/O logics and pass gate switches 62 ineach pass gate switch array 61. This is because there are fewer signalscoming in at any given time that need to be processed by the pass gateswitches 62. Depending on the type of interleaving, there would be thesame number of I/O logics and pass gate switches 62 in each pass gateswitch array 61 as there are bus lines 33 in the W bus 32.

In the read operation, the process described above is essentiallyreversed for the bits of the ATM cell to ultimately reach the registercells 57 of the registers 56 of a CDU 50. A discussion about DRAMinterleaving can be found in:

EDN Mar. 30, 1989 pages 155 to 166;

EDN Apr. 13, 1989 pages 157 to 164;

EDN Apr. 27, 1989 pages 183 to 188;

EDN May 11, 1989 pages 179 to 186; all of which are incorporated byreference

The apparatus 10 can also be used as a switch, cell rate multiplexer orcell rate demultiplexer. The mode in which the apparatus 10 is used isdetermined by the mode register 126. The mode register 128 is connectedto the W-state machine 48 the CVUs 46 and the CDUs 50 to allow them tocarry out the transfer of the cells according to the mode designated bythe mode register 126. The mode register 126 can control whether theCVUs 46 operate individually, all together, or in various groupings.Similarly, the mode register 126 also controls the CDUs 50 to cause themto operate individually, all together or in groupings.

The mode register 126 receives mode information from an external pin orpins. This information is written into the mode register 126 by a mode₋₋strobe 134 signal pin. The mode register 126 is loaded with theinformation before the operation of the apparatus 10 begins, e.g., atthe initialization time. The mode register 126 is not changed during theoperation of the apparatus.

Furthermore, depending on the coding, the mode register 126 wouldspecifically indicate how to structure the relationship between the CVUsand CDUs. For CVUs, the mode register specifies following encodings.

000 All the four CVUs working individually.

001 CVU0, CVU1 are grouped together. CVU2, CVU3 working individually.

010 CVU2, CVU3 are grouped together. CVU0, CVU1 working individually.

011 CVU0, CVU1 are grouped together. CVU2, CVU3 grouped together.

100 All the CVUs grouped together.

Similarly, for CDUs, the mode register specifies following encodings.

000 All the four CDUs working individually.

001 CDU0, CDU1 are grouped together. CDU2, CDU3 working individually.

010 CDU2, CDU3 are grouped together. CDU0, CDU1 working individually.

011 CDU0, CDU1 are grouped together. CDU2, CDU3 grouped together.

100 All the CDUs grouped together.

Along with the mode of operation information the mode register 126contains T-count information for each CVU 46. The T-count information isfive bits wide and it indicates to the CVU 46 the length of the ATM celldata and any additional data that need to be stored in terms of bytes.The T-count can have the following decodings:

    ______________________________________                                               00000         48 bytes                                                        00001         49 bytes                                                        00010         50 bytes                                                        00011         51 bytes                                                        00100         52 bytes                                                        00101         53 bytes                                                        00110         54 bytes                                                        00111         55 bytes                                                        01000         56 bytes                                                        01001         57 bytes                                                        01010         58 bytes                                                        01011         59 bytes                                                        01100         60 bytes                                                        01101         61 bytes                                                        01110         62 bytes                                                        01111         63 bytes                                                        10000         64 bytes                                                 ______________________________________                                    

Thus, for the CVUs the mode register contains 23 bits of information; 3bits specifying the mode of operation and 5 bits of T-count for eachCVU.

Along with the mode of operation information the mode register containsT-count information for each CDU 50. The T-count information is fivebits wide and it indicates the CDU 50 the length of the ATM cell dataand any additional data that will be received from W bus in terms ofbytes. The T-count can have following encodings:

    ______________________________________                                               00000         48 bytes                                                        00001         49 bytes                                                        00010         50 bytes                                                        00011         51 bytes                                                        00100         52 bytes                                                        00101         53 bytes                                                        00110         54 bytes                                                        00111         55 bytes                                                        01000         56 bytes                                                        01001         57 bytes                                                        01010         58 bytes                                                        01011         59 bytes                                                        01100         60 bytes                                                        01101         61 bytes                                                        01110         62 bytes                                                        01111         63 bytes                                                        10000         64 bytes                                                 ______________________________________                                    

Thus, for the CDUs the mode register contains 23 bits of information. 3bits specifying the mode of operation and 5 bits of T-count for each CDU50. Therefore, in this embodiment, the mode register 126 contains 46bits of information.

The mode register 126 causes the apparatus 10 to operate as a switch, acell rate multiplexer or a cell rate demultiplexer. Following are someexamples of their operation in various modes.

In one example of 4×4 switch operation, CVU mode 000 and CDU mode 000,the apparatus 10 receives ATM cells on four inputs at 155 megabits(OC-3) input rate, stores the ATM cells into the memory array 12 andthen reads the ATM cells on W bus 32 as requested by the CDUs. Then theCDU 50 send the ATM cells on the corresponding output port.

In one example of the cell rate multiplexer operation, CVU mode 000, CDUmode 100, the apparatus 10 takes ATM cells on four inputs at 155Megabits (OC-3) inputs rate and outputs ATM cells at 622 Megabits(OC-12) rate. In this case, each CVU 46 receives a complete ATM cell andwrites it into the memory array 12. When the cell is received by the CDU50, the 1/4 cell is written to each CDU 50 in the following way.

Byte 1 is written into first layer of output register 0 of CDU0.

Byte 2 is written into first layer of output register 1 of CDU1.

Byte 3 is written into first layer of output register 2 of CDU2.

Byte 4 is written into first layer of output register 3 of CDU3.

Byte 5 is written into first layer of output register 4 of CDU0.

Byte 6 is written into first layer of output register 5 of CDU1.

Byte 7 is written into first layer of output register 6 of CDU2.

Byte 8 is written into first layer of output register 7 of CDU3.

and so on.

In case the CDU outputs ATM cells on a 4 byte wide path at 622 Megabits(OC-12).

In one example of the cell rate demultiplexer operation, CVU mode 001,CDU mode 000, the apparatus 10 takes ATM cells at 622 Megabits (OC-12)input rate and outputs ATM cells at 155 Megabits (OC-3) rate over fouroutputs. In this case, each CVU 46 receives a 1/4 ATM cell as shownbelow.

Byte 1 is written into first layer of input register 0 of CVU0.

Byte 2 is written into first layer of input register 1 of CVU1.

Byte 3 is written into first layer of input register 2 of CVU2.

Byte 4 is written into first layer of input register 3 of CVU3.

Byte 5 is written into first layer of input register 4 of CVU0.

Byte 6 is written into first layer of input register 5 of CVU1.

Byte 7 is written into first layer of input register 6 of CVU2.

Byte 8 is written into first layer of input register 7 of CVU3.

and so on.

The W-state machine 48 enables the tri-state enables associated withbytes from all the CVU 46 such that a valid ATM cell is constructed onthe W-Bus 32. The W-state machine 48 writes the cell into the memoryarray 12. Each CDU 50 receives a complete ATM cell and outputs it at 155Megabits (0C-3) rate.

As can be seen by the mode encoding a mixed operation as switch, cellrate multiplexer and cell rate demultiplexer is possible.

In the above example, each CVU 46 and CDU 50 interface with acommercially available ATM physical layer part (not shown), such as partnumber PM5346 manufactured by PMC Sierra. This part is commonly known asSunilite. Sunilite is 155 Megabit (OC-3) physical layer part.

In this regard, the apparatus 10 can also act in a multicast mode, wherethe mode register 126 would cause each CDU 50 to operate individuallybut receive a copy of an ATM cell that is received at a CVU 46 and thenstored in the memory sub-array 16. After it is stored in the memorysub-array 16, it then is copied as many times is needed in distinct CDUswhich are each associated with a different connectivity.

The present invention pertains to a method for switching an ATM cell.The method comprises the steps of receiving the ATM cell at a firstinput port of a switch from the ATM network. Then there can be the stepof storing the ATM cell in one clock cycle in a memory array 12 of theswitch. Next there is the step of reading the ATM cell in the memoryarray 12 in one clock cycle. Next there is the step of transferring theATM cell from the memory array 12 to a first output port of the switch.Then there is the step of transmitting the ATM cell from the firstoutput port to the ATM network 36.

The switch can be preferably the apparatus 10 described above. The firstinput port can be comprised of one or many CVUs as described above. Thestoring step can be the step of providing the ATM cell onto the W bus 32and providing it to the memory array 12 as described above. The readingstep can be the step of obtaining the bits of the ATM cell with theaddress information from a CDU as described above. The step oftransferring the ATM cell from the memory array 12 to a first outputport can be the step of transferring the bits of the ATM cell onto the Wbus 32 to the registers 56 of the CDU 50 or CDUs 50 as described above.The first output port can be comprised of one or more CDUs 50 dependingupon the application. The step of transmitting the ATM cell to the ATMnetwork 36 can be the step of transferring bit by bit out of theregisters 56 of a CDU or CDUs 50 through the register 129 as describedabove.

If a multicast mode is chosen by the mode register 126, then after thetransferring step, there is the step of reading again the ATM cell inthe memory array 12 in one clock cycle. Next there is the step oftransferring the ATM cell from the memory array 12 to the first outputport of the switch. Then there is the step of transmitting the ATM cellfrom the first output port to the ATM network 36. In this way, the ATMcell can be copied as many times as desired and sent out the same CDU ordifferent CDUs 50 to different locations to achieve the multicast.

If the mode register 126 chooses a multiplexing mode for the apparatus10, then the receiving step can include the step of receiving J portionsof the ATM cell at J corresponding CVUs 56. The J CVUs 46 comprise thefirst input port. The transferring the ATM cell step can include thesteps of transferring K portions of the ATM cell to corresponding K CDUs50. In this instance, J≧1≦K and J and K are integers. The K portions ofthe ATM cell comprise the ATM cell. The K CDUs 50 define the firstoutput port. Thus, the ATM cell arrives at J CVUs and transmittedthrough K CDUs, where K>J.

In the event the apparatus 10 is used as a demultiplexer, the receivingthe ATM cell at a first input port includes the steps of receiving Pportions of the ATM cell at corresponding P CVUs 46. Here, P≧2 and is aninteger. The P portions of the ATM cell comprise the ATM cell. The PCVUs 46 define the first input port. The transferring step then includesthe step of transferring Q portions of the ATM cell to Q CDUs 50. Here,Q≧1≦P and Q is also an integer. Q portions of the ATM cell comprise theATM cell and Q CDUs 50 comprise the first output port. Thus, fordemultiplexing, P portions of the ATM cell arrive at P CVUs 50. The Pportions are subsequently transferred out of the apparatus 10 through QCDUs 50 in Q corresponding portions, where P>Q.

The present invention pertains to a switch for an ATM cell. The switchcomprises I input ports which receive ATM cells from an ATM network 36,where I≧1 and is an integer. The switch is also comprised of a memoryarray 12 connected to the I input ports for storing an ATM cell receivedby one of the I input ports in one clock cycle. The switch alsocomprises O output ports connected to the memory array 12, where O≧1 andis an integer. One of the O output ports transmit an ATM cell which isreceived from the memory array 12 to the ATM network 36. Additionally,the switch comprises a controller connected to the memory array 12, Iinput ports and O output ports for controlling the storage of an ATMcell from one of the input ports into the memory array in one clockcycle.

The switch can be the apparatus 10 as described above. The I input portscan be the CVUs in various combinations as described above. The 0 outputport can be the CDUs 50 in various combinations as described above. Thecontroller can be comprised of an external controller 110 as describedabove and an internal controller. The internal controller can includethe W state machine 48, for instance, and also, for instance, the moderegister 126. Depending on the configuration, the internal controllercan also include the reading or writing mechanism as described abovesuch as a row decoder mechanism and a mechanism for selecting a memorysub-array.

If the switch is used for multicast, the controller causes the ATM cellto be copied at least two times and transmitted to one of the outputports. Preferably, one of the output ports is comprised of Q CDUs 50,where Q≧1 and is an integer. In this way, the ATM cell is copied atleast two times and multicast out through the same CDU 50 or severalCDUs 50 to accomplish the multicast.

The switch can operate to simply move or transfer one ATM cell receivedat an input port through the memory array and out through a desired CDUof an output port to cause the cell to be moved on towards its desireddestination.

If the switch is used as a demultiplexer, then preferably one of theinput ports is comprised of P CVUs 46 which receive P correspondingportions of the ATM cell. Here, P≧2 and is an integer, and the Pportions comprise the ATM cell. Additionally, one of the O output portsis comprised of Q CDUs 50, where 1≦Q≦P. The controller causes the Pportions of the ATM cell to be stored in one clock cycle in the memoryarray 12, then read in one clock cycle from the memory 12 into Qportions and transfer the Q portions of the ATM cell to Q correspondingCDUs 50. In this way, the P portions of the ATM cell are transmitted outthe Q CDUs where P>Q.

If the switch is used as a multiplexer, one of the input ports iscomprised of J CVUs 46 which receive J corresponding portions of the ATMcell. Here, J≧1 and is an integer, and the J portions comprise the ATMcell. Preferably, one of the O output ports is comprised of K CDUs 50.Here, J≧1≦K. The controller causes the P portions of the ATM cell to bestored in one clock cycle in the memory array 12, then read in one clockcycle from the memory array 12 into K portions and transfer the Kportions of the ATM cell to K corresponding CDUs 50. In this way, the Jportions of the ATM cell are transmitted out of the switch as K portionswhere J<K, and multiplexing is obtained.

Although the invention has been described in detail in the foregoingembodiments for the purpose of illustration, it is to be understood thatsuch detail is solely for that purpose and that variations can be madetherein by those skilled in the art without departing from the spiritand scope of the invention except as it may be described by thefollowing claims.

What is claimed is:
 1. A method for multicasting an ATM cell comprisingthe steps of:receiving the ATM cell at a first input port of a switchfrom an ATM network; storing the ATM cell in one clock cycle in a memoryarray of the switch; reading the ATM cell in the memory array in oneclock cycle; transmitting the ATM cell from the switch to the ATMnetwork; reading again the ATM cell in the memory array in one clockcycle; and transmitting again the ATM cell from the switch to the ATMnetwork.
 2. A method as described in claim 1 including after the readingthe ATM cell step, there is the step of transferring the ATM cell to afirst output port of the switch.
 3. A method as described in claim 2including after the reading again the ATM cell step, there is the stepof transferring again the ATM cell to the first output port of theswitch.
 4. A method as described in claim 2 including after the readingagain the ATM cell step, there is the step of transferring again the ATMcell to a second output port of the switch.
 5. A multicast switch for anATM cell comprising:a first input port which receives the ATM cell froman ATM network; a memory array connected to the first input port forstoring the ATM cell in one clock cycle; a first output port connectedto the memory array, said ATM cell is transmitted to the ATM networkthrough the first output port; and a controller connected to the memoryarray, first input port and second output port for controlling thestorage of the ATM cell in the first input in the memory array in oneclock cycle, and the number of N copies, where N≧2 and is an integer, ofthe ATM cell that are provided to the first output port, said controlleralso produces the clock cycle.
 6. A multicast switch as described inclaim 5 including a second output port connected to the controller andthe memory array, said ATM cell is transmitted to the ATM networkthrough the first output port, said controller controlling the number ofcopies of the ATM cell in the memory array that are provided to thesecond output port.
 7. A multicast switch as described in claim 6wherein the controller includes an external controller which producesthe clock cycle and address information regarding the ATM cell in thememory array, said external controller connected to the input and outputports and memory array.
 8. A multicast switch as described in claim 7wherein the controller includes an internal controller that controls theinput and output ports and the memory array, and is connected therewith.9. A multicast switch as described in claim 8 wherein the internalcontroller includes a W-state machine connected to the input and outputports and the memory array.
 10. A multicast switch as described in claim9 wherein the input port includes a CVU.
 11. A multicast switch asdescribed in claim 10 wherein the output port includes a CDU.
 12. Amethod for switching an ATM cell comprising the steps of:receiving Jportions of the ATM cell at J corresponding CVU's, said J CVU's comprisea first input port of a switch from an ATM network; storing the ATM cellin one clock cycle in a memory array of the switch; reading the ATM cellin the memory array in one clock cycle; transferring K portions of theATM cell from the memory array to corresponding K CDU's, where 1≦J<K andJ and K are integers, said K portions of the ATM cell comprising the ATMcell, said K CDU's defining a first output port of the switch; andtransmitting the ATM cell from the first output port to the ATM network.13. A method as described in claim 12 wherein the receiving the ATM cellat a first input port includes the steps of receiving P portions of theATM cell at corresponding P CVU's, where P≧2 and is an integer, said Pportions of the ATM cell comprising the ATM cell, said P CVU's definingthe first input port.
 14. A method as described in claim 13 wherein thetransferring step includes the step of transferring Q portions of theATM cell to Q CDU's, where 1≦Q<P and Q is an integer, and Q portions ofthe ATM cell comprising the ATM and Q CDU's comprising the first outputport.
 15. A method as described in claim 12 including after thetransferring step, there is the step of reading again the ATM cell inthe memory array in one clock cycle, transferring the ATM cell from thememory array to the first output port of the switch; and transmittingthe ATM cell from the first output port to the ATM network.
 16. A switchfor an ATM cell comprising:I input ports which receive ATM cells from anATM network, where I≧1 and is an integer; a memory array connected tothe I input ports for storing an ATM cell received by one of the I inputports in one clock cycle; O output ports connected to the memory array,where O≧1 and is an integer, one of the 0 output ports transmit an ATMcell which is received from the memory array to the ATM network; and acontroller connected to the memory array, I input ports and O outputports for controlling the storage of an ATM cell from one of the inputports into the memory array in one clock cycle, said controller causesthe ATM cell to be copied at least two times and transmitted to one ofthe output ports.
 17. A switch as described in claim 16 wherein one ofthe output ports is comprised of Q CDU's where Q≧1 and is an integer.18. A switch as described in claim 16 wherein one of the input ports iscomprised of P CVU's which receive P corresponding portions of the ATMcell, where P≧2 and is an integer, and the P portions comprise the ATMcell; and wherein one of the 0 output ports is comprised of Q CDU's,where 1≦Q<P; and wherein the controller causes the P portions of the ATMcell to be stored in one clock cycle in the memory array, then read inone clock cycle from the memory array into Q portions and transfer the Qportions of the ATM cell to Q corresponding CDU's.
 19. A switch asdescribed in claim 16 wherein one of the input ports is comprised of JCVU's which receive J corresponding portions of the ATM cell, where J≧1and is an integer, and the J portions comprise the ATM cell; and whereinone of the 0 output ports is comprised of K CDU's, where 1≦J<K; andwherein the controller causes the P portions of the ATM cell to bestored in one clock cycle in the memory array, then read in one clockcycle from the memory array into K portions and transfer the K portionsof the ATM cell to K corresponding CDU's.
 20. A method of demultiplexingan ATM cell comprising the steps of:receiving P portions of the ATM cellat P corresponding CVU's which serve the ATM cell from an ATM network,said ATM comprised of the P portions, where P≧2 and is an integer;storing the ATM cell in one clock cycle in a memory array of the switch;reading the ATM cell in the memory array in one clock cycle; andtransferring Q portions of the ATM cell from the memory array to Qcorresponding CDU's which transmit the ATM cell to an ATM network, where1≦Q<P.
 21. A method of multiplexing an ATM cell comprising the stepsof:receiving J portions of the ATM cell at J corresponding CVU's whichserve the ATM cell from an ATM network, said ATM comprised of the Jportions, where J≧1 and is an integer; storing the ATM cell in one clockcycle in a memory array of the switch; reading the ATM cell in thememory array in one clock cycle; transferring K portions of the ATM cellfrom the memory array to K corresponding CDU's which transmit the ATMcell to an ATM network, where 1≦J<K.